Patch: adding custom node socket shapes to the Node Editor

Currently, node socket shapes are hard-coded as circles. This patch allows alternative shapes to be drawn, as controlled by a draw_shape() callback in a custom NodeSocket Python class. Defining this callback is optional.

Draw_shape() takes the same arguments as draw_color(). It must return an enum. This patch defines this enum as "CIRCLE" (SOCK_SHAPE_CIRCLE) or "DIAMOND" (SOCK_SHAPE_DIAMOND). 
SOCK_SHAPE_CIRCLE is associated with the current drawing function. For SOCK_SHAPE_DIAMOND, a new drawing function has been added in space_node/node_draw.c. More shapes can easily be added later if desired.

The definition of draw_shape() is optional. If it is not defined (NULL), a circle is drawn. Existing behavior is therefore maintained in all cases.

[lukas.toenne1, 2013-08-02 09:45:21]

https://codereview.appspot.com/12245043/diff/1/source/blender/makesrna/intern...
File source/blender/makesrna/intern/rna_nodetree.c (right):

https://codereview.appspot.com/12245043/diff/1/source/blender/makesrna/intern...
source/blender/makesrna/intern/rna_nodetree.c:2240: static void
rna_NodeSocketStandard_draw_shape(ID *id, bNodeSocket *sock, struct bContext *C,
PointerRNA *nodeptr, int *r_shape)
In this case it can just be a simple return value, no need to pass a r_ pointer.
Was just needed for color returns, because float[4] cannot be a simple return.

[sjoerd_de_vries, 2013-08-02 10:28:35]

patch v2, updated as per message #2